A measurement or determination of a morphological property of an optic disc and/or a retinal nerve fiber layer of an eye serves inter alia the purpose of recognizing glaucoma. Glaucoma always leads to a loss of nerve fibers in the area of the optic disc or in the area of an exit site of an optic nerve, respectively. This manifests in a progressive excavation of the optic disc, which is also called optic nerve head. As a result of the loss of nerve fibers, degenerations of the field of vision, or so-called scotoma, occur. Thus, glaucoma leads to morphological defects because of damage to nerve pathways and to functional defects in the form of scotoma.
Scotoma can be determined by an examination of the field of vision or by perimetric measurements, respectively. In a perimetric examination of a field of vision of an eye, a patient is exposed to optical stimuli, the patient giving feedback as to whether he recognized a stimulus or not. For instance, perimeters are known in which a patient looks into a hemisphere or a flat screen, via which, controlled by a computer, spots of light are projected in different positions and at various levels of brightness. In this way, it is possible to examine both the overall extent of a visual field and the condition of selected regions thereof. A brightness of the points of light may be varied in such manner that a threshold value above which the patient detects the point of light can be determined for a position on the retina.
Various methods for determining visual fields and threshold values are known from the state of the art. In essence, a series of stimuli with various gradations of brightness are directed at a point on the retina to be measured or tested, so that the specific threshold value for the point may be determined. In order to localize a threshold value, this may be carried out with uniform or non-uniform gradations of stimuli, for example. Additionally, a database containing representative threshold values of an individual person may be incorporated. For example, a possible threshold value may be localized within a threshold range taking into account a patient's age and sex. It is also known that there is a relationship between directly adjacent regions or measured points in the retina. For instance, threshold values typically do not differ substantially between neighboring points. Accordingly, a threshold value for a directly adjacent point may be sought within a probable threshold value range. It is an objective of the known perimetric methods to determine a field of vision as precisely as possible with a number of stimuli as low as possible being provided to a patient. From EP 2 361 547 A1, for example, a perimetry method is known in which a complete measurement of the field of vision can be carried out with the help of a very low number of stimuli or measured points of an eye's retina. Also, a series of other, different perimetric methods are known by means of which a determination of a field of vision is possible. However, all the aforementioned methods only provide a determination of functional data of a field of vision because it is a subjective measurement of a recognition of stimuli by the patient.
Morphological defects in the area of the retinal nerve fiber layer, optic disc or optic nerve head can be determined and measured by means of imaging methods, such as Confocal Heidelberg Retinal Tomography (HRT), Scanning Laser Polarimetry (GDx) and Optical Coherence Tomography (OCT). For example, an OCT measurement allows a depth measurement of the retina and also measuring, in the area near to the optic nerve head or optic disc, a thickness of a sheet or layer of optic nerves leaving the optic nerve head. In the course of this, inter alia, a reduction of a neuroretinal ring can be measured as well.
Primarily, it is desirable to be able to recognize the development of glaucoma at an early stage. While morphological and functional defects can occur simultaneously, it is also possible that morphological defects are recognizable before any functional effects occur. Nevertheless, functional defects always have the underlying cause of morphological defects. The functional consequences of morphological defects, however, are individually different so that a direct link cannot always be established. For instance, a measured nerve fiber layer thickness in a person can be without functional effects and thus lie in a normal range, whereas the same nerve fiber layer thickness in another person does already lead to functional degenerations. The nerve fibers form a compact mass, thus morphological damage to one area cannot be associated to only one area of the field of vision. Also, such a morphological defect can have an impact on multiple areas of the field of vision. Even in a more detailed analysis of an area of the field of vision it is unknown which of the analyzed points have a more or less close connection to the morphological damage. Hence, only imprecise conclusions concerning scotomas can be drawn from an objective measurement of an optic disc by means of an imaging method since scotomas are subject to individual variations. Conversely, while there are always morphological defects when there are functional defects, it is excluded for the afore-mentioned reasons that a statement concerning morphological defects is possible at all with sufficient accuracy on the basis of functional defects.
Nevertheless, the afore-mentioned imaging methods for measuring an optic disc are especially useful for the early diagnosis of glaucoma because they can detect even minimal damage to the optic disc. It is, however, a disadvantage of the imaging methods that the respective ophthalmological devices are much more expensive than the devices used for perimetric methods. An OCT measurement of an optic disc is thus much costlier than a perimetric measurement of a field of vision.
Therefore, it is the task of the present invention to propose a method and an ophthalmological device which allows a determination of morphological defects in the area of the optic disc with sufficient accuracy.